1. Field Of The Invention
The present invention generally concerns a unit for amplifying light signals in optical fiber transmission lines, and, more specifically, to such a unit which reduces interference and noise in such lines and which is connected between a pair of optical fibers.
2. Description Of The Prior Art
As known, in the optical fiber telecommunications field, owing to the unavoidable losses of light occurring inside optical fibers, a gradual attenuation of the signal always takes place along the path of the optical fibers.
For this reason, when signals have to be transmitted long-distances, it is necessary to use one or more amplifying units which are interposed along the path of the optical fibers at intervals of pre-fixed length.
A type of amplifying unit that at present is in widespread use provides for the use of an optical fiber amplifier which in operation is connected to the optical fibers so as to define, along the path of the latter, an input line through which the signals of light are transmitted to the amplifier itself, as well as an output line through which the amplified signals of light are transmitted in the direction of an optical receiver.
At the present state of the technique, the use of these optical fiber amplifiers gives rise to some drawbacks, deriving mainly from the fact that the amplifier receives not only and exclusively the useful signal to be amplified, but also different noise signals which are consequently amplified and introduced again into the output line.
It is found that a certain number of these noise signals come from the output line and are caused by a phenhomena of diffusion of light unavoidably arising inside the optical fibers.
More precisely, a part of the light forming the amplified signals gets lost as a result of a phenomenon of diffusion arisinq inside the optical fibers.
A part of the back-diffused light returns to the amplifiers and, therefore, is again amplified and introduced into the output line.
Moreover, it is to be considered that the amplifier, owing to its intrinsic nature, emits a certain quantity of noise signals which are introduced either into the input line or into the output line.
Owing to the above phenomena of diffusion, these noise signals partly come back to the amplifier where they mix with the useful signals which it is desired be transmitted.
As it can be understood from the above, the entry of noise signals into the amplifier and their consequent amplification result in interferences and beat phenomena that, for sufficiently high values (&gt;15 dB) of gain of the amplifier, originate an "interferometric noise" of an amplitude greater than the known noise produced by the amplifier.
The above problem results in an undesired reduction of the signal-to-noise ratio between the useful signal and the noise downstream of the amplifier itself. This reduction in the signal/noise ratio tends to increase by increases in the gain of the fiber optical amplifier, as well as by increasing the number of amplifiers arranged along the path of the fibers.
In this situation it is extremely difficult to have the useful signal sufficiently clear when it reaches a receiver placed at a long distance from the source of the signal itself.
From Japanese patents 52-155901 and 63-219186 and from "ELECTRONICS LETTERS", vol. 24, no. 1, Jan. 7, 1988, pages 36-38, it is known that in a laser or in an optical semiconductor amplifier there is the risk of instability and generation of oscillations due to the reflections at the amplifier ends.
In the above patents and article, in order to eliminate these reflections, it is taught to couple an optical isolator to the semiconductor laser, which prevents the light reflected by the coupling surfaces between the line fibers and these devices from reaching the lasers themselves.
In an active-fiber amplifier no interface surfaces are present between the line fibers and the amplifier because the line fibers are directly welded to the amplifier's active fiber. Therefore, the reflection phenomena are not generally expected.
It has, however, been discovered that in an active-fiber amplifier, in the absence of means for limiting reflections towards the active fiber, it is impossible to reach high amplification gain due to the occurrence of noise of the interferometric type as a result of beats between the direct and reflected signals in the line fibers themselves and in any event directed towards the active fiber. The presence of interferometric noise is of little importance in a semiconductor amplifier which has low gains and small construction sizes, whereas it becomes particularly important in an active-fiber amplifier capable of reaching very high gains and having an active fiber of considerable length generally in the range of some tens of meters, much greater than the coherence distance of the signal generating laser.
In an optical fiber amplifier the problem arises, therefore, of protecting the amplifying fiber against such noise sources and keeping the reflections towards the active fiber itself below critical values so as not to jeopardize the transmission quality, while maintaining high values of amplification gain.